This invention relates to cleaning of objects that relate to semiconductor printing, such as, for example, screening masks. This invention is basically directed to removing, for example, an organic polymer-metal composite paste from screening masks used in printing conductive metal pattern onto ceramic green sheet in the fabrication of semiconductor packaging substrates. More particularly, this invention is concerned with automated in-line cleaning of paste screening masks with aqueous alkaline solution of a quaternary ammonium hydroxide as an environmentally friendly alternative to non-aqueous organic solvents based cleaning in screening operations for the production multilayer ceramic (MLC) substrates.
In the fabrication of multilayer ceramic substrates for packaging semiconductor devices, conductive metal patterns are screened on individual ceramic green sheets by printing, such as, extrusion printing, using mask, such as, a metal mask, placed in contact with the green sheet. The screen printing method involves squeeging paste onto the green sheet surface using a mesh mask or an emulsion mask. After screening, the green sheets-are assembled and aligned, and laminated followed by a sintering operation to form a multilayer ceramic substrate. Fabrication techniques for such substrates, including design, screening equipment, and paste screening process are well known in the art.
However, advance ground rule electronic packaging requires printing closely spaced conductive metal pattern on a substrate, and using a screening mask that has highly dense fine dimension etched features. It has been observed that such screening masks have the problem of paste residue entrapment in the mask features in addition to a surface residue when the paste is screened to deposit conductive pattern onto the green sheets. This requires that the metal mask be cleaned after one or more screening passes to eliminate/minimize the possibility of defects in subsequently screened pattern. Any defects in the screened paste pattern replicates into the final product causing yield losses. On the other hand, mesh masks can be used for multiple screening passes, typically 100-200 green sheets may be screened using the same mesh mask before it requires cleaning and therefore, the cleaning frequency for a mesh mark is relatively low in comparison to a metal mask.
Conductive pastes used in screening processes for the delineation of wiring, visa, I/O (input-output) pads, and other pattern features comprise metal particles with or without added inorganic particulate fillers. These constituents are mixed with an organic binder and solvent vehicle along with wetting agents, dispersants/surfactants, plasticizers, and other additives such as thickening agents, antioxidants, and coloring agents which are well known in the fabrication of electronic components.
Most commonly used conductive pastes in multilayer ceramic fabrication are based on molybdenum or tungsten metal powders dispersed in an organic polymer binder, such as, for example, ethyl cellulose, polymethylmethacrylate, and the like, or polyhydrocarbon based thermoplastic resins in a high boiling solvent vehicle.
Other conductive pastes that can be used for screening processes in multilayer ceramics could comprise copper, gold, nickel, tin, to name a few. Because of the variety and complex chemical make-up of polymer-metal pastes, it is required that the mask cleaning medium and process selected be such that it provides complete and efficient cleaning of all types of pastes from screening masks and associated equipment.
In automated processes for high volume production of multilayer ceramic substrates, it is also necessary that the speed of mask cleaning be compatible with the cycle time pre-set by the thruput requirement and other dependent operations.
Recently, several water-based cleaners have become commercially available and many aqueous detergent compositions have been described for cleaning/degreasing purposes, for example, printed circuit board assemblies and other parts in device fabrication to remove soldering flux, oil/grease, and other organic residues invariably formed during bonding and assembly processes in microelectronics. These are generally based on a combination of surfactants in water and/or alkaline detergent compositions comprising alkali metal salts, such as, sodium metasilicate, sodium carbonate, tribasic sodium phosphate, sodium tripolyphosphate, and combinations thereof, as well as highly alkaline solutions based on alkali metal salts, alkali metal hydroxides, and mixtures thereof with alkanolamines. Aqueous cleaner formulations comprising this category of alkaline detergents are described in U.S. Pat. No. 5,234,506 (Winston) and U.S. Pat. No. 5,264,047 (Winston) for removing soldering flux, wax, and grease from printed circuit boards.
One of the major considerations with aqueous cleaning is wastewater management for compliance with the Clean Water Act regulations, which requires that all possible sources of aquatic impact and other environmentally undesirable constituents be removed from the effluent before discharge to surface waters. This requires special and costly wastewater treatments, such as, precipitation with complexing agents to form sludge that carries active ingredients of cleaning compositions, metals, and other dissolved constituents, and which results in solids loading to the waste. To minimize waste and reduce cost with conservation of water in aqueous cleaning, there is currently significant industry direction toward environmentally suitable zero waste processes by devising a methodology for reclamation, recycling, reuse, etc.
U.S. patent application Ser. No. 09/096,849 filed on Jun. 12, 1998, entitled xe2x80x9cREMOVAL OF SCREENING PASTE RESIDUE WITH QUATERNARY AMMONIUM HYDROXIDE-BASED AQUEOUS CLEANING COMPOSITIONSxe2x80x9d, assigned to the assignee of the instant Patent Application, and the disclosure of which is incorporated herein by reference, discloses the use of quaternary ammonium hydroxide-based aqueous cleaning compositions as a more environmentally friendly medium for removing screening paste residue.
The invention is a novel method of cleaning paste from screening masks using at least one aqueous quaternary ammonium hydroxide as a cleaner.
Therefore, it is a purpose of this invention to provide a water-based cleaning method for cleaning paste residue from screening masks used in the manufacturing of multilayer ceramic substrates.
Another purpose of this invention is to provide an aqueous cleaning method for automated in-line cleaning of screening masks, in multi-layer ceramic manufacturing in high volume production.
Yet another purpose of this invention is to provide aqueous cleaning method for paste cleaning from screening masks, by using water-based solution of a quaternary ammonium hydroxide to eliminate volatile emissions and hazardous wastes associated with non-aqueous solvent based cleaning processes.
Still yet another purpose of this invention is to provide method for in-line mask cleaning with aqueous alkaline solution comprising a quaternary ammonium hydroxide as a single active ingredient which can be recovered and recycled.
Still another purpose of this invention is to provide aqueous alternative to organic solvents in mask cleaning by using an aqueous alkaline solution containing tetramethyl ammonium hydroxide that is effective in removing polar, non-polar, or hydrophilic and hydrophobic paste residue from metal and polymer surfaces.
It is also a purpose of this invention to provide an aqueous cleaning method for screening masks which does not require special wastewater treatment.
Yet another purpose of this invention is to provide an aqueous cleaning method for paste screening masks which provides an option for metals recovery from the effluent and minimizes waste in multilayer ceramic production.
Still another purpose of this invention is to provide an aqueous cleaning method using quaternary ammonium hydroxide containing compositions which are compatible with metal masks, emulsion masks, polymer adhesives used in mask assembly, and all contacting materials in cleaning tools associated with paste making equipment.
Still yet another purpose of this invention is to provide a cleaning method with an aqueous alkaline solution which is free of foaming problems when used under high pressure spray conditions.
Therefore, in one aspect this invention comprises an aqueous cleaning method for cleaning paste residue from at least one screening object comprising the use of at least one aqueous alkaline solution containing at least one quaternary ammonium hydroxide.
In another aspect this invention comprises a cleaning apparatus comprising:
(a) at least one means for pressurized spray cleaning of at least one screening object carrying screening paste residue with at least one solution; and
(b) wherein said at least one solution comprises at least one aqueous alkaline composition having at least one quaternary ammonium hydroxide.